1. Field of the Invention
This invention relates to electronic volume circuits for electrically adjusting signal levels and particularly, to an electronic volume circuit that can be driven at a low voltage and by a single power source.
2. Prior Art
Latest versions of volumes for audio equipment and the like are designed to implement remote control by adding a motor-driven shaft rotating function to conventional volumes that are operated by manually rotating the shafts of rotary variable resistors. The volumes of this type are generally provided with as many rotary variable resistors as the number of output channels, which are mechanically linked with each other. To adjust the sound volume of signals involving many channels such as surrounding music, the number of rotary variable resistors for linkage needs to be increased with an increase in the number of channels. When the number of rotary variable resistors for linkage increases, larger torque is required to rotate these resistors, thus increasing not only power for driving the motor but also the size of the volume itself. It is also difficult to operate all the rotary variable resistors for linkage with identical characteristics. To overcome these problems, electronic volumes have been developed and commercialized, which are designed to adjust sound volume by means of an electronic circuit.
FIG. 1 is a block diagram showing an example of the arrangement of a conventional electronic volume circuit. In the figure, the electronic volume circuit includes input terminals 1 and 2, series-connected resistors 6 having the same resistance value with each other, a multiplexer 7, a control circuit 8, and an amplifier 9. An input signal Si is supplied to the electronic volume circuit via the input terminal 1 and a DC cutoff capacitor 4. The multiplexer 7 selects voltages at the junctions between the resistors 6 based on 8-bit control data from the control circuit 8, and outputs the selected voltages to the amplifier 9. The amplifier 9, which is driven by two power sources of xc2x15V, amplifies the selected voltages from the multiplexer 7 at an amplification factor determined by the values of resistors 10 and 11, and outputs the amplified voltages through an output terminal 12. The control circuit 8 converts volume control data (serial data) supplied through a terminal 13 into parallel data, and delivers the parallel data to the multiplexer 7.
The above described conventional electronic volume circuit requires two power sources of xc2x15V. Therefore, a power circuit for such power sources is more complicated than that for a single power source, and if the electronic volume circuit is formed of an LSI, a complicated process is required for fabricating such an LSI.
This invention has been made in view of these circumstances, and an object thereof is to provide an electronic volume circuit that can be driven by a single power source and can therefore be formed by an LSI that can be fabricated in a simple manner using an oxide film and a junction process for a single power source.
To attain the above object, the present invention provides an electronic volume circuit comprising a first amplifying circuit that attenuates amplitude of a bipolar input signal and converts the attenuated input signal to a unipolar signal, and a variable resistor device that controls a degree of attenuation of the first amplifying circuit based on an externally supplied signal.
According to this construction, the electronic volume circuit can be driven by a single power source, and can therefore be formed by an LSI that can be fabricated in a simple manner using an oxide film and a junction process for a single power source.
In a preferred form of the present invention, the electronic volume circuit according to the present invention comprises a first amplifying circuit that attenuates amplitude of a bipolar input signal and converts the attenuated input signal to a unipolar signal, a variable resistor device that controls a degree of attenuation of the first amplifying circuit based on an externally supplied signal, and a second amplifying circuit that amplifies an output from the first amplifying circuit and converts the amplified output to a bipolar signal.
Preferably, the variable resistor device comprises a plurality of resistors connected in series with each other, and a selecting circuit that selects voltages at junctions between the plurality of resistors based on the externally supplied signal.
Also preferably, the electronic volume circuit according to the present invention comprises a capacitor connected to an input signal source having an end, that supplies the input signal, and a DC voltage source that supplies a predetermined fixed DC voltage, and the first amplifying circuit comprises an operational amplifier that operates as an inverting amplifier, the operational amplifier having a non-inverting input thereof connected to the end of the input signal source through the capacitor as well as to the DC voltage source, and the second amplifying circuit comprises a differential amplifier that amplifies a difference between an output from the operational amplifier and a signal applied to the non-inverting input of the operational amplifier.
According to this construction, the electronic volume circuit can be free of noise derived from grounding resistance.
Further preferably, the electronic volume circuit according to the present invention further comprises a third amplifying circuit that shunts a current flowing to the variable resistor device.
According to this construction, an electronic volume circuit is provided which can implement high-gain and hence multilevel control using low-gain amplifiers.
The above and other objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.